1. Field of the Invention
The invention relates to a disc reproducing apparatus, a disc recording and/or reproducing apparatus, and a motor control circuit of the disc recording and/or reproducing apparatus. More particularly, the invention relates to a disc reproducing apparatus, a disc recording and/or reproducing apparatus, and a motor control circuit of the disc recording and/or reproducing apparatus, in which the reading operation of data from a disc and the recording operation of data to the disc are intermittently executed.
2. Description of the Related Art
A recording and/or reproducing apparatus for compressing digital audio data and recording and/or reproducing the compressed data to an optical disc or magnetooptic disc having a diameter of, for example, 64 mm enclosed in a disc cartridge is prevailing. In the recording and/or reproducing apparatus, the digital audio data is compressed and recorded onto the optical disc or magnetooptic disc, thereby enabling the digital audio data of a recording time which is almost equivalent to a CD (Compact Disc) to be recorded by using the foregoing optical disc of a diameter of 64 mm. As a compression system of digital audio data, a system for compressing data on the basis of MDCT (Modified Discrete Cosine Transform) is used.
In the recording and/or reproducing apparatus, upon reproduction, the recording data recorded on the optical disc is read out from the optical disc and the read-out recording data is subjected to an EFM demodulation and an error correcting process and, after that, the processed data is once accumulated in a buffer memory. The digital data read out from the buffer memory is sent to a decoder and is decompressed.
A transfer rate of the data read out from the optical disc when the reproduction is performed by the foregoing recording and/or reproducing apparatus is equal to a rate of 1.4 Mbits/sec. On the other hand, a data transfer rate which is necessary in the decoder is equal to 0.3 Mbits/sec. Therefore, the data read out from the optical disc is written into the buffer memory at a rate of 1.4 Mbits/sec. and is read out from the buffer memory at a rate of 0.3 Mbits/sec. Since the transfer rate of the data read out from the optical disc is higher than the reading rate of the data from the decoder, the data read out from the optical disc is successively accumulated into the buffer memory. At a time point when a certain amount of data has been stored in the buffer memory, the reading operation of the data from the optical disc is stopped and the reading operation of the data from the buffer memory is continued. When an amount of data stored in the buffer memory is smaller than a predetermined amount, the reading operation of the data from the optical disc is restarted and the read-out data is written into the buffer memory as mentioned above.
In the foregoing recording and/or reproducing apparatus, the reading operation of the data from the optical disc is intermittently performed. For a period of time during which the data of a predetermined amount or more has been stored in the buffer memory and the data is not read out from the optical disc, only the reading operation of the data from the buffer memory and the decoding process of the data read out from the buffer memory are executed. Therefore, in case of intermittently reading out the data from the optical disc, for a period of time during which the reading operation from the optical disc is not performed, it is sufficient to make operative only a portion of the reading process of the buffer memory, a portion of a decoding process, and a portion of a D/A converting process for converting the reproduction data into an analog signal and outputting, and the other circuit portions, namely, a driving circuit of a laser beam, a servo circuit, a demodulating circuit, and the like can be turned off. By turning off the other circuit portions as mentioned above for a period of time during which the reading operation of the optical disc is not performed, an electric power consumption can be reduced by an amount corresponding to such a turn-off state.
Data has been recorded at a constant linear velocity on the optical disc or magnetooptic disc which is used in the foregoing recording and/or reproducing apparatus. When the data is read out from the optical disc or magnetooptic disc, it is necessary to control a rotational speed of the disc to a constant linear velocity. When the reading operation of the disc is not executed as mentioned above, the control such as to keep the rotational speed of the disc at a constant linear velocity is unnecessary. When the reading operation of the disc is not performed, a method of reducing the electric power consumption by stopping a power supply to a spindle motor for rotating the disc or turning off the servo of the spindle motor is considered.
However, if the rotation of the optical disc or magnetooptic disc is stopped when the reading operation of the data from the optical disc or magnetooptic disc is not performed, in case of restarting the rotation of the optical disc or magnetooptic disc, a large current is needed as a driving current to activate the spindle motor, so that an electric power consumption contrarily increases. If the spindle servo of the spindle motor is turned off, the rotational speed of the optical disc or magnetooptic disc becomes unstable for such a turn-off period of time and a speed difference between such an unstable rotational speed and the rotational speed of the optical disc or magnetooptic disc while the servo is turned off when the rotation of the optical disc or magnetooptic disc is restarted increases. Therefore, a large current is necessary as a driving current to drive the spindle motor until the rotational speed of the optical disc or magnetooptic disc is controlled to a predetermined rotational speed, contrarily resulting in an increase in electric power consumption. In the foregoing recording and/or reproducing apparatus, therefore, upon reproduction, the reading operation of the optical disc or magnetooptic disc is intermittently performed. When data is read out from the optical disc or magnetooptic disc, the disc is controlled at a constant linear velocity. When the optical disc or magnetooptic disc is not read out, it is desired to control the optical disc or magnetooptic disc at a constant angular velocity.
There is considered a method of controlling in a manner such that the optimum angular velocity is obtained by an arithmetic operation from an address on the optical disc or magnetooptic disc from which data is being read out, and when the operating mode is switched from the reading mode of the optical disc or magnetooptic disc to a mode in which the reading operation of the optical disc or magnetooptic disc is not performed, the rotational speed of the optical disc or magnetooptic disc is controlled to a constant angular velocity in accordance with the angular velocity obtained by the arithmetic operation.
For example, FIG. 1 shows an example of a conventional servo circuit in case of rotating an optical disc 101 at a constant angular velocity when data is not read out from the optical disc 101. As shown in FIG. 1, a servo circuit comprises: a period measuring circuit 111 for detecting a rotational speed of a spindle motor 102 and measuring a period of an FG signal as a detection signal; a reference value register 112; a comparing circuit 113; a low pass filter 114; and a driver 115. A reference value arithmetic operating circuit 116 for operating an optimum reference value for the reference value register 112 is provided. An address is supplied from a system controller (not shown) to the reference value register 116 through a terminal 117.
An FG signal is generated from the spindle motor 102 in accordance with the rotational speed of the spindle motor 102. The FG signal is supplied to the FG period measuring circuit 111. The FG period measuring circuit 111 measures the period of the FG signal. An angular velocity of the spindle motor 102 is detected from the period of the FG signal. An output of the FG period measuring circuit 111 is supplied to the comparing circuit 113.
An output of the reference value register 112 is supplied to the comparing circuit 113. An output of the reference value arithmetic operating circuit 116 is supplied to the reference value register 112. The reference value arithmetic operating circuit 116 calculates an optimum reference value on the basis of the address which is supplied from the system controller (not shown) through the input terminal 117.
That is, if the address of the data reading position on the optical disc or magnetooptic disc is known, the distance of such a position in the radial direction from the center of the optical disc or magnetooptic disc can be known. Therefore, the rotational speed at the foregoing reading position in case of rotating the optical disc or magnetooptic disc at the constant linear velocity can be obtained from the distance in the radial direction from the center of the optical disc or magnetooptic disc and the angular velocity of the optical disc by an arithmetic operation. On the basis of the result of the arithmetic operation, the reference value of the rotational speed is outputted from the reference value arithmetic operating circuit 116. An output of the reference value arithmetic operating circuit 116 is supplied to the reference value register 112.
The comparing circuit 113 compares the period of the FG signal serving as a reference and stored in the reference value register 112 with the present period of the FG signal of the spindle motor 102, thereby obtaining a speed error and a phase error. An output of the comparing circuit 113 is supplied to the spindle motor 102 through the low pass filter 114 and driving circuit 115.
As mentioned above, the optimum angular velocity of each address of the optical disc or magnetooptic disc is obtained by the reference value arithmetic operating circuit 116 and when the operating mode is switched from the reading mode of the optical disc or magnetooptic disc to the mode where the reading operation of the optical disc or magnetooptic disc is not performed, by controlling the rotational speed of the optical disc or magnetooptic disc so as to be a constant angular velocity in accordance with the optimum angular velocity obtained by the arithmetic operation as mentioned above, it is possible to prevent the occurrence of a speed difference between the rotational speed of the optical disc or magnetooptic disc when the reading operation of the optical disc or magnetooptic disc is not performed and the rotational speed when the reading operation of the optical disc or magnetooptic disc is restarted.
However, the linear velocity of the optical disc or magnetooptic disc in the foregoing recording and/or reproducing apparatus has a variation in a range from 1.2 m/sec to 1.4 m/sec. That is, the linear velocity is equal to 1.4 m/sec in the magnetooptic disc which can record data corresponding to 60 minutes or the optical disc on which data as much as 60 minutes has been recorded. The linear velocity is equal to 1.2 m/sec in the magnetooptic disc which can record data as much as 72 minutes or the optical disc on which data as much as 74 minutes has been recorded. As mentioned above, in case of obtaining the optimum angular velocity corresponding to the address on the disc and controlling the optical disc or magnetooptic disc so as to rotate at a constant angular velocity, the reference value of the rotational speed is obtained from the address and the angular velocity of the disc by the reference value arithmetic operating circuit 116 by an arithmetic operation. Therefore, as mentioned above, in case of obtaining the optimum angular velocity corresponding to the address on the disc and controlling the rotational speed so as to be the constant angular velocity, the optimum angular velocity cannot be obtained unless the variation in linear velocity as mentioned above is considered.
In the foregoing recording and/or reproducing apparatus, the writing operation of the magnetooptic disc is intermittently performed even upon recording. Even in case of intermittently performing the writing operation to the magnetooptic disc as mentioned above, it is desirable to control so as not to cause a speed difference between the rotational speed when the writing operation of the magnetooptic disc is not performed and the rotational speed of the magnetooptic disc when the writing operation of the magnetooptic disc is restarted.
When the control mode of the optical disc is switched from the rotating state of the constant linear velocity to the rotating state of the constant angular velocity, in case of setting the rotational speed of the disc so as not to cause a difference between the rotational speeds of the optical disc, the rotational speed of the optical disc when the rotation is controlled at the constant angular velocity differs depending on the scanning position of the optical head. In the foregoing recording and/or reproducing apparatus, although a gain of a CAV servo circuit is constant, when the rotational speed of the optical disc differs, the gain of the servo loop changes. It is, therefore, desirable to control the gain of the CAV servo circuit in accordance with the rotational speed of the optical disc.